The rest of the mould is self-explanatory with what has gone
before, and is entirely formed by loam boards.
Rapping plates have become a necessity in order to prevent
injury to the pattern by the moulder.
They are shewn at Fig. 734 being let into
the pattern, and are screwed to receive a
lifting rod as there shown.
^ Crystallization of cast iron. —
During the cooling of a casting the
crystals arrange themselves in lines per-
pendicular to the surface, but the interior
portion, being cooled more slowly, pre-
serves its granular nature. Fig. 74 will
shew the appearance of a bar of cast
iron when broken longitudinally (the
student should clearly understand that
the markings are exaggerated).
If the corners of the casting are made
quite sharp the crystals will be abruptly
turned at these places, and, meeting each
other also abruptly for some distance
below the surface, namely, as far down
as they are formed, will create a line of
fracture or portion weaker than the rest. Whether these corners
be external or internal, matters not; -the same thing happens.
Fig. 75 shows other examples having 're-entrant angles/ as they
are called, A being a circular boss cast on a plate, and B a cylinder
with flanges. It will be clear that breakage would always occur
more easily at these sharp angles.
When the Menai Bridge was built, the hydraulic press made •
for the purpose of lifting the * tubes; had a flat bottom with
pretty sharp corners, as will be understood from Fig. 76, which is
a sketch of the press first used. Stephenson took the precaution
of building up at each 10 inches of lift, and, had it not been for
this, great damage might have occurred, for the bottom of *the
press suddenly gave way, and the tube fell through a space of ten
inches. Fig. 77 represents the press since adopted, the crystals being
allowed to take a gradual turn, so as to leave no line of fracture.